There is known a fuel cell system that includes: a gas-liquid separator storing and separating water from a fuel gas partially discharged from a fuel cell; and a discharge valve connected to the gas-liquid separator and partially discharging the fuel gas to the outside together with the stored water in the gas-liquid separator. For example, Japanese Unexamined Patent Application Publication No. 2005-302708 describes technology for estimating a discharge amount of the fuel gas by opening the discharge valve. In some cases, the discharge valve herein includes a discharge outlet opened and closed by a valve body.
To finely estimate the discharge amount of the fuel gas, it is preferable to finely estimate a discharge flow rate of the fuel gas per unit time. The discharge flow rate of the fuel gas is finely estimated conceivably based on, for example, a differential pressure between upstream and downstream sides of the discharge valve during the opening period of the discharge valve. The discharge flow rate is estimated conceivably by use of the detected differential pressure on the basis of, for example, the relationship between the differential pressure and the gas discharge flow rate that is defined beforehand through experimental results.
The estimation of the gas discharge flow rate based only on the differential pressure might, however, degrade the estimation accuracy. For example, although the fuel gas is partially discharged after the stored water is discharged by opening the discharge valve, water generated by electric generation of the fuel cell might be discharged as the stored water from the gas-liquid separator through the discharge outlet of the discharge valve to the outside even during the discharge of the fuel gas. At this time, the stored water and the fuel gas are conceivably discharged from the discharge outlet at the same time. A percentage of an area, through which the fuel gas substantially flows, to a cross sectional area of the discharge outlet is herein by subtracting a percentage of the stored water to the cross sectional area of the discharge outlet therefrom. The percentage of the stored water to the cross sectional area of the discharge outlet varies with the amount of the stored water. Thus, the percentage of the area, through which the fuel gas substantially flows, to the cross sectional area of the discharge outlet varies. Therefore, if the gas discharge flow rate is estimated based only on the differential pressure without considering the percentage of the area, through which the fuel gas substantially flows, to the cross sectional area of the discharge outlet, the estimation accuracy of the gas discharge flow rate might deteriorate.